1. Field of the Invention
The present invention relates to a chip electronic component including a ceramic element and terminal electrodes having a metal coating disposed on the ceramic element.
2. Description of the Related Art
In recent years, needs for surface mountable electronic components have been increasing and therefore needs for chip electronic components have also been increasing. For example, a monolithic PTC thermistor is manufactured according to the following procedure: ceramic green sheets containing BaTiO3 and conductive paste containing conductive powder such as Ni are alternately laminated, and sintered integrally to form a ceramic element. Then, terminal electrodes containing Ag or the like are each formed on the end faces of the ceramic element by a printing process.
A chip electronic component, manufactured according to the above procedure, is normally soldered when it is mounted on a substrate. During the soldering step, terminal electrodes of the component are in some cases partly melted into the solder (this phenomenon is so-called xe2x80x9csoldering erosionxe2x80x9d) when the soldering temperature is higher than the melting point of the terminal electrodes or when the soldering time is extremely long. In order to prevent soldering erosion, the following method has been employed: metal coatings containing Ni or the like are formed on the corresponding terminal electrodes by an electroplating process before the soldering step.
However, when a ceramic element has a small sintered density, even if it is insulative, a plating solution permeates into the ceramic element and deteriorates its properties. Furthermore, when terminal electrodes sintered onto a semi-conductive ceramic element are subjected to electroplating, there is a problem that metal coatings are also formed on portions of the element on which the terminal electrodes are not disposed.
In order to solve the above problems, the following method has been proposed: a ceramic element is soaked in a sodium-silicate, in which the atomic ratio of sodium to silicon is 0.6, to form a glass layer on the ceramic element (refer to Japanese Unexamined Patent Application Publication No. 2002-43167, pp., for example). Formation of such an insulative glass layer on the ceramic element prevents metal coatings from being formed on it. Furthermore, it has been thought that, by use of glass powder inclusive of a large amount of one of the alkali metal elements, the difference in shrinkage between the ceramic layer and the glass layer can be reduced, thereby preventing cracks in the glass layer, because a melting point of glass is lower than usual.
However, a chip electronic component having a glass layer thereon containing a large quantity of one of the alkali metal elements has proved to drop extremely in the dielectric strength according to the energization test.
Accordingly, an object of the present invention is to provide a chip electronic component where a sufficient dielectric strength is secured, and at the same time, a plating solution is prevented from permeation into a ceramic element due to generation of cracks in a glass layer when a glass layer is formed on the surface of a ceramic element.
In order to solve the above problems, a first preferred embodiment of the chip electronic component of the present invention includes a ceramic element and terminal electrodes having a metal coating thereon disposed on the surface of the ceramic element. A glass layer is disposed on at least portions of the ceramic element surface on which the terminal electrodes are not formed. The glass layer is made of a glass material containing at least two species of alkali metal elements selected from Li, Na and K, and the atomic total amount of the alkali metal elements is greater than or equal to 20 atomic percent of the atomic total amount of elements, except oxygen.
In this chip electronic component, formation of cracks in the glass layer and deterioration in its dielectric strength can be prevented. More specifically, when a large amount of alkali metal elements are contained in a glass material to form a glass layer, alkali metal elements are ionized and therefore ionic conduction takes place, whereby an electric current is allowed to flow on the surface of the chip electronic component and the dielectric strength is seriously deteriorated. Therefore, the inventors noticed that, from the fact that ion conduction takes place via positions exclusively occupied by alkali ions, by using two or more species of alkali ions to occupy two or more different positions in advance, the migration of the alkali ions is prevented. According to this configuration, even if a glass material to form a glass layer contains a large amount of alkali metal elements, the dielectric strength of a chip electronic component can be prevented from being deteriorated. Furthermore, the amount of the alkali elements contained in a glass material can be increased compared to the prior art and therefore the melting point of the glass layer can be lowered, which can prevent cracks in a glass layer more efficiently.
In the above chip electronic component, as a second preferred embodiment of the present invention, the above at least two species of alkali metal elements preferably include at least Li and K. When a combination of Li and K is employed as the two or more alkali metal elements, ionic conduction can be more securely prevented because of a large difference between the ionic radii of a Li ion and a K ion. Furthermore, this case of chip electronic component, which can be manufactured at low cost without deterioration in its properties, is suitable for mass-production.
In a third preferred embodiment of a chip electronic component of the present invention, among alkali metal elements contained in a glass layer, the atomic ratio of two species of alkali metal elements having a highest ranking much preferably falls in the range from 2:8 to 8:2. By control of the ratio of the alkali metal elements contained in the glass layer within this range, the above ionic conduction can be more efficiently prevented and a remarkable effect of suppression of deterioration in the dielectric strength can be brought out.
According to a fourth preferred embodiment of the present invention, the above ceramic element preferably contains a semi-conductive ceramic material. This case is more useful because metal coatings, which are readily formed on a ceramic element when the ceramic element is made of a semi-conductive ceramic material, can be prevented from being formed thereon due to the configurations according to the first or second preferred embodiment of the present invention.